OnTriggerEnter2D does not work on inactive objects?

On key press, Object A makes Object B active and on key up, makes it inactive. Everytime Object B is active, I need to check for collisions on Object B for which I have the following code. I’m not getting any collisions being detected. What am I doing wrong ?

using UnityEngine;
using System.Collections;

public class DestroyRadius : MonoBehaviour {

		private void OnTriggerEnter2D(Collider2D kaboom)
			if (kaboom.gameObject.tag == "Crowd")
				Debug.Log("Object B is colliding with crowd");

When a GameObject is in active, it is as if it doesn’t exist. Other than directly calling methods of scripts attached to this object, or changing it’s properties directly from another script, there is not much you can do with it.

Some of the things that won’t work on an inactive GameObject: No physics calculations (including movement, collisions, forces, etc), it will not be rendered, no lifecycle calls (update, ongui, fixedupdate).

So yes, in order for the object to get collision calls it needs to be active. Consider disabling only the renderer, then the object will appear but will not be visible to the camera:

renderer.enabled = false;